Refactor RelativePatcher out of OatWriter.
Move the relative patcher classes to compiler/linker/ and
compiler/linker/<arch>/ . Refactor them to avoid OatWriter
dependency so that they can be unit tested. Add tests for
x86 and x86-64.
Change-Id: I1b42baa9fc431378e4cce1399bec590c5b5a409f